The present invention relates to a rotary shaft of an aircraft turbojet, which shaft is supported in rotation by rolling bearings. The invention relates more particularly to a system enabling such a rotary shaft to be decoupled in the event of it being subjected to a large unbalance.
A two-spool bypass turbojet has two rotors, namely a low-pressure rotor and a high-pressure rotor, which rotors are supported by rolling bearings. Typically, the low-pressure rotor is supported by two rolling bearings at the front (referred to herein as “bearing 1” and “bearing 2”) and by a rolling bearing at the rear. These bearings need to be able to withstand the axial and radial loads of the turbojet.
In such a turbojet, the breaking of a fan blade (e.g. as a result of ingesting a bird) gives rise to an unbalance on the low-pressure shaft in the plane of the fan. In such a situation, large loads associated with the resulting unbalance are transmitted to the structures of the turbojet, firstly via the bearing 1 that supports the shaft close to the fan, and secondly as a result of contacts between the fan blades and the casing that surrounds them. These loads must therefore be taken into account when designing the turbojet. Reducing them makes it possible to reduce the weight of the turbojet.
In order to achieve this, it is known to install a decoupling system for the bearing 1, and also to increase the clearance between the tips of the fan blades and the casing surrounding them so as to reduce considerably the loads that are transmitted to the structures in the event of a large unbalance on the low-pressure shaft. Typically, a decoupling system is in the form of fuse bolts or columns that are rated to break at a certain load, thereby eliminating the connection between the low-pressure shaft and the stationary support for the bearing 1 in the event of a large unbalance on the low-pressure shaft. This reduces the resonant frequency of the suspension of the fan in the operating range of the low-pressure shaft, thereby reducing loads at high speed. Having recourse to a decoupling system is nevertheless effective only if contacts between the fan blades and the casing that surrounds them are reduced, and that requires a large amount of clearance between those parts.
Furthermore, the drop in the frequency of the suspension mode of the fan as a result of the fuse bolts breaking also gives rise to a change in the nature of that suspension mode, which tends to deform the low-pressure shaft rather than the stationary support of the bearing 1. That gives rise to significant bending of the low-pressure shaft under the high-pressure rotor of the turbojet. In particular, contacts may occur between the low-pressure shaft and the high-pressure shaft, which contacts increase the risk of the low-pressure shaft breaking.
A known way of limiting such inter-shaft contacts during the deceleration that results from a fan blade breaking is to add a decoupling system to the bearing 2. That decoupling system, which is rated to break after the decoupling system of bearing 1, serves to release radial clearance at the bearing 2 in order to limit inter-shaft contacts. A mechanical abutment situated at bearing 2 serves advantageously to limit the radial movements of the low-pressure shaft after the decoupling system of this bearing 2 has broken.
Nevertheless, such a solution with two decoupling systems presents the drawback whereby the increase in clearance at bearing 2 as a result of breakage of the decoupling system allows the low-pressure rotor to start orbiting during stages of autorotation operation (“windmilling”) and contributes to increasing vibration levels at low speed.